Using celestial bodies and landmarks to obtain orientation has been used for many years to orient travelers, particularly in ships and in other vessels and vehicles. Also, compasses have been used for the purpose of orientation. In addition, compasses have been used in combination with the sighting of celestial bodies to determine orientation.
Compasses are not accurate in many instances due to interference from nearby magnets, magnetic devices and conductors. This is particularly true with respect to the use of magnets in vehicles such as cars and trucks since there is quite a lot of metal in and around the vehicle and since vehicle's typically contain magnetic devices and electrical devices that interfere with the measurement of magnetic orientation.
Though sighting of celestial bodies and landmarks has been used for centuries for determining orientation, prior art systems for determining orientation of vehicles using celestial bodies and landmarks only determine a rough orientation. Typically such orientation methods involve the sighting the celestial object optically so as to determine a rough estimate of north, south or west directions. More sophisticated methods used by mariners included the use of a sextant which was used to sight stars for a more exact orientation.
Recently, in-vehicle navigation systems have been used which determine position and travel path of the vehicle using position determining signals transmitted from satellites. These navigation systems typically include a receiver which receives position determination signals from satellites, a microprocessor, a display and a map database. Typically, the position determination signals are received from global positioning system (GPS) satellites which are a part of the US GPS satellite network. The position determination signals are processed via an electronics package located within the receiver or by the microprocessor unit.
These systems typically include a display screen which displays a map including travel paths such as streets, roads and highways. Many of these systems display the location of the vehicle as an icon on the display and show the movement of the vehicle by moving the icon on the display. These navigation systems typically determine orientation by calculating the direction of movement of the vehicle. However, the GPS system does not provide orientation information. Orientation is predicted using previously obtained position data.
Position determining systems can accurately predict orientation when the vehicle is moving rapidly in a straight line when signals from multiple satellite vehicles are received in an uninterrupted fashion. However, when obstructions and/or atmospheric conditions and/or selective availability either do not allow for the calculation of position, or introduce error into the position determination calculation, orientation is either totally unavailable or the calculated orientation is incorrect.
Dead reckoning systems have been incorporated into GPS systems for determining orientation when orientation determined by the GPS system is either incorrect or is unavailable. Dead reckoning systems work well in situations where position determination is unavailable or incorrect for short periods of time. However, such systems typically use gyroscopic devices to determine orientation. The gyroscopic devices used in GPS systems drift over time. Thus, as time passes, the orientation becomes less and less accurate. FIG. 1 shows an example of a vehicle 1 which includes a vehicle navigation system with dead reckoning. Vehicle 1 is shown to be traveling along street A at the intersection of street A and street B. As vehicle A continues to travel along street A, the orientation and position determined using the dead reckoning system drifts, showing the position to be outside of the actual travel path as illustrated by dashed line 2. In the example shown in prior art FIG. 1, a new position is obtained at the intersection of street A and street C. The new position is then used to calculate direction of travel which is used to correct for gyroscopic drift. However, the drift of the gyroscope will continue to degrade the accuracy of the determined orientation and the determined position as shown by dashed line 4 until another new position is accurately obtained and correction of gyroscopic drift is made as shown by line 5. Though orientation is shown to be determined accurately by the movement of the vehicle as represented by arrow 4 and arrow 5, often this is not the case. For example, even when the vehicle's position is determined with accuracy, the direction of travel of the vehicle may be incorrect since it is based on the assumption that the vehicle will move in the same direction as it has been traveling. Thus, if the vehicle is in the process of turning or has turned at, street C or street D, the correction to the drift of the gyroscope will be wrong. The error in drift will then continue to compound until such time that a sufficient number of positions have been determined so as to establish a new travel path.
Orientation is also a problem in surveying using optical sighting systems. Though optical sighting systems provide good data with regard to the orientation of a marker or an object relative to a reference site, the orientation of the reference site is typically not known with accuracy. For example, though the use of a plumb-bob for determining the levelness of the optical instrument can provide a rough vertical orientation (attitude), such a measurement does not provide the accuracy required for many applications. With reference to azimuthal orientation, a compass is typically used to obtain an orientation to true north. The azimuthal reference obtained by the use of a compass is also inexact due to variations in the earth's magnetic field and magnetic deviation due to surrounding metal and electrical objects and devices. The height of the position is also unknown with any degree of accuracy unless references having a known height are nearby.
Recently position determination systems such as GPS have been used for orienting optical surveying systems. However multiple GPS systems are required to determine orientation and each GPS system must be able to accurately determine position in order to determine a sufficiently accurate height and a reference attitude and azimuthal orientation.
What is needed is an accurate orientation system. In particular, a orientation system is required which accurately determines orientation and position when satellite signals are blocked or are inaccurate. In addition, an orientation system which can orient an optical surveying system when satellite signals are blocked or are inaccurate is required. In addition, an orientation system that can work in conjunction with a position determination system to provide orientation as a supplement to GPS data is needed.